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Old October 18th, 2002, 10:47 AM   #1
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Audio question: cable coils

We don't seem to have an audio forum, so I'm posting this here since the camera in question is a GL2. This is a newbie question, to be sure.

I'm using external mics with balanced lines into an Studio One Pro box, then the camera. Normally, I tempt fate and leave the extra cable length loose on the floor, but I'll be going on a location shoot this weekend in which it would make a lot more sense to coil any unused cable length. Does coiling mic cables, or the feed cable from the Studio One box to the camera, create any kind of problem. I know in general that coils=inductance and blockage of AC (signal) voltages, but it seems to me the number of turns and the current levels so low that it probably doesn't matter.

As I said, a newbie question, and probably a good laugh to those who know, but I'd rather ask than find out editing later!

Thanks in advance for your opinions.
Linc Kesler
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Old October 18th, 2002, 01:54 PM   #2
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I would urge against it, except in the case of overall safety. It increases the chance of picking up RF, ground loop hum etc.

Jeff
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Old October 19th, 2002, 06:42 AM   #3
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The inductance of any coil you are likely to make with mic cable will not be an issue for audio signals. However, best to keep cable short as you can, especially the cable onthe unbalanced sice (between the GL1 and the adapter). Buying a second set of shorter mic cables is cheaper than a retake if you have a noise problem, and makes for a neater set. As to pickup of signals, if the cable is quality balanced cable, the change of stray pickup from a coil of the extra lenght is lsight.
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Old October 19th, 2002, 05:12 PM   #4
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Coiling cables is not a problem as long as you do it the right way: bifilar. Take the center of the excess cable and start coiling with the two parts togheter.
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Old October 22nd, 2002, 11:04 AM   #5
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Dre,
This sounds like a great tip, but I can't tell what you mean. I've wondered about this issue as well, and I'd really appreciate it if you could make another stab at describing this technique. On our shoot, excess cable is a real hassle.
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Old October 22nd, 2002, 01:04 PM   #6
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OK I will give it a try... Balanced interconnections are in principle immune for, or at least strongly suppress, "external fields". This is true as long as the common mode induced voltages don't exceed the common mode rejection limit of the input amps. Normally this is not a problem like Don already wrote, but in the presence of strong fields, and big coils (many turns), these voltage can become quite substancial. However, the induced voltages cancel each other if one half of the excess cable is being wind-up clock wise and the other half counter clock wise and both halfs are (magnetically) closely coupled. To do this, start at the middle of the excess cable and start winding with both parts( the part towards the mile and the part towards the camcorder) together (bi-filar = two wires) together. By doing this you realize the above. Bifilar winding techniques are often used in audio transformers when tight coupling is required. The bifilar winding also cancels interference is unbalanced (screened wire) interconnections.
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Old October 23rd, 2002, 10:51 AM   #7
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Thanks!

Appreciate it. I think I got it.
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